With advances in semiconductor manufacture, the density of integrated circuits on semiconductor dice has increased. The higher circuit density increases the power consumption of the dice, and also the heat generated by the dice. In order to dissipate heat, semiconductor packages are sometimes constructed with heat sinks for dissipating heat. One type of heat sink comprises a metal plate bonded to a face or backside of a semiconductor package. The plate can be surface mounted on the package or embedded in an encapsulant material of the package.
In the fabrication of electronic assemblies, it is sometimes necessary to attach the heat sink of a semiconductor package to a substrate, such as printed circuit board (PCB), or a multi chip module substrate. Electrical connections may also be required between the heat sink and the substrate. For example, some heat sinks require a path to ground. Typically, these electrical connections comprise solder joints bonded to the heat sink, and to corresponding contacts on the substrate. In addition to providing electrical paths, the solder joints provide thermally conductive paths between the heat sink and substrate, for dissipating heat by conduction to the substrate.
One problem with solder joints is that stresses can develop in the solder material due to differences in thermal expansion between the heat sink and substrate. Typically the heat sinks are formed of metals, such as copper, having relatively high coefficients of thermal expansion (CTE). On the other hand, the substrates are typically formed of materials, such as glass filled resins or ceramics, having a different coefficient of thermal expansion.
These stresses can cause cracks which weaken the solder joints. In some situations the joints can loosen and fail. In addition, the cracks can increase the electrical resistivity of the joints, and decrease the thermal conductivity through the joint. In view of the foregoing, improved semiconductor packages with heat sinks, and improved methods for fabricating semiconductor packages and electronic assemblies with heat sinks are required.